246 Darwin, and after Darwin. 
ness to leave no manner of doubt as to their affinities 
with the fauna of that mainland. Lastly, no am- 
phibians and no mammals (except bats) are ever 
found on any oceanic islands. Yet, as we have seen, 
on the theory of special creation, these islands must 
all be taken to have been the theatres of the most 
extraordinary creative activity, so that on only three 
of them we found no less than 1258 unique species, 
whereof 657 were unique species of land animals, to 
be set against one single species known to occur else- 
where. Nevertheless, notwithstanding this prodigious 
expenditure of creative energy in the case of land- 
birds, land-shells, insects, and reptiles, no single new 
amphibian, or no single new mammal, has been 
created on any single oceanic island, if we except 
the only kind of mammal that is able to fly, and 
the ancestors of which, like those of the land-birds 
and insects, might therefore have reached the islands 
ages ago. Moreover, with regard to mammals, 
even in cases where allied forms occur on either 
side of a sea-channel, it is found to be a general rule 
that if the channel is shallow, the species on either 
side of it are much more closely related than if it be 
deep—and this irrespective of its width. Therefore 
we can only conclude, in the words of Darwin—* As 
the amount of modification which animals of all kinds 
undergo partly depends on lapse of time, and as the 
islands which are separated from each other or from 
the mainland by shallow channels are more likely to 
have been continuously united within a recent period 
than islands separated by deeper channcls, we can 
understand how it is that a relation exists between 
the depth of the sea separating two mammalian 
